Ceramic flake formation in the aluminosilicate system by plasma spraying

Alumina and aluminosilicate flakes with compositions Al₂O₃, 3Al₂O₃2SiO₂ and Al₂O₃2SiO₂ have been produced from commercial raw materials using a direct current plasma spray process in air. The microstructure and phase constitution of the as-sprayed flakes were examined with optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Changes in phase constitution of the flakes with heat-treatment were examined using differential thermal analysis (DTA) and XRD of heat-treated samples. The as-sprayed Al₂O₃ flakes consisted of -Al₂O₃ phase plus a minor -Al₂O₃ phase. The -phase could be removed by heat treating the flakes at 1300C for 2 h. The aluminosilicate flakes consisted of 3Al₂O₃:2SiO₂ (mullite) and an amorphous phase which crystallised to 3Al₂O₃:2SiO₂ (mullite) after heat treatment at 1100C for 2 h. These flakes may find applications as high temperature thermal insulation materials.